The present invention relates to a spark plug.
Conventionally, spark plugs are used in internal combustion engines such as gasoline engines and gas engines. Japanese Laid-Open Patent Publication No. 2017-103179 discloses a spark plug of the type having an auxiliary combustion chamber. In this spark plug, a cap is fixed to a front end part of a metal shell such that the auxiliary combustion chamber is defined within the cap. A hole is formed in the cap so as to provide communication between the auxiliary combustion chamber and the outside. An air-fuel mixture is introduced into the auxiliary combustion chamber through the hole of the cap. Further, a center electrode and a ground electrode are disposed in the auxiliary combustion chamber. When a spark discharge is generated in a gap between the center electrode and the ground electrode, the air-fuel mixture introduced into the auxiliary combustion chamber is ignited by the spark discharge. A flame is developed upon the ignition, and propagates to the outside, that is, to the combustion chamber of the internal combustion chamber through the hole of the cap. By such flame propagation, the air-fuel mixture in the combustion chamber is burned.